The invention relates to digital vehicle speed recorders for recording the speed of a vehicle over a predetermined distance, and in particular to such recorders comprising a pulse generator connected with a velocity sensor in the vehicle for generating control pulses with a pulse separation corresponding to a given road length, and having means for providing speed values on the basis of said control pulses and internally generated clock pulses, and a speed value memory for storing said speed values with a recording rate dependent on the distance covered as well as read-out means associated with the memory for connection with a reproducing instrument for obtaining a visual presentation of the speed values stored in the memory.
Hitherto, the use of such recorders to obtain a presentation of the speed of a vehicle over a given road length preceding a certain reading time, for example in case of traffic accidents, has mainly been limited to large vehicles like trucks and busses. In prior art devices having built-in, at least partly mechanically operating curve-drawing instruments, the recorders have been relatively complicated and, in addition, sensitive to mechanical influences during operation, so that they have often been rather inefficient with respect to producing a speed presentation in connection with an accident.
In more recent devices of the kind mentioned, such as described, for instance, in German Pat. Nos. 2,929,168 and 3,123,879, the problems caused by mechanically operating parts have been remedied through a fully electronic data storage in the recording device installed in the vehicle itself in combination with a separate external reproducing instrument, by means of which a visual speed presentation may be produced by connecting the instrument to the recording device in the vehicle and reading out the data recorded therein.
However, the devices known from the above mentioned publications suffer from the disadvantages that they have either been relatively complicated with considerable demands on storage capacity of the electronic memory due to a desire to record operational parameters other than the speed, for instance a clock indication for each record, the number of revolutions and consumption of fuel, or offer only the possibility of a speed presentation for a very limited road length due to the memory design itself.
The apparatus described in German Pat. No. 2,929,168 uses a semiconductor memory having three series-connected FIFO-shift registers for storing pulses from a road distance sensor together with time indicators in the first location in the first shift register. For each pulse, the time indication is advanced in the first shift register, and at the transfer to the next and the following shift registers a data depletion is accomplished in that such later shift registers are clocked with lower frequencies than the shift frequency for the first shift register, for instance the half or one fifth of this frequency, whereby only every second or every fifth entry will be transferred from one shift register to the next. In practice, with this memory design the speed reconstruction or presentation will be limited to a road length of some hundred meters.
In the apparatus according to German Pat. No. 3,123,879, a buffer-controlled CMOS-memory is used and in addition to speed, data is stored for several operational parameters with a time indication associated with each record. The data concentration in the memory is accomplished in this case by a discontinuous storage, whereby new data are only recorded if they have either changed to a predetermined extent since the preceding entry, or a predetermined maximum road distance between two entries has been exceeded. The microprocessor used as calculating unit must be programmed to perform the comparison operations necessary therefore on several levels.